1. Field of the Invention
The present invention relates to color photography and more particularly, it relates to a novel class of two-equivalent magenta couplers which are suitable for use in silver halide color photographic light-sensitive materials.
2. Description of the Prior Art
It is known that, by the color development of a silver halide color photographic material, a color developing agent of aromatic primary amine type is oxidized and is reacted with a coupler to form a dye, such as an indophenol, an indoaniline, an indamine, an azomethine, a phenoxazine, a phenazine and the like, thus forming a color image. In this type, the subtractive color process is ordinarily used for color reproduction and yellow, magenta and cyan color images are formed, which are respectively the complementary colors of blue, green and red. For example, a coupler of the acylacetanilide or dibenzoylmethane type is used for forming a yellow color image, a coupler of the pyrazolone, pyrazolobenzimidazole, cyanoacetophenone or indazolone type is used for forming a magenta color image and a coupler of the phenol type, such as a phenol and a naphthol, is used for forming a cyan color image.
In one of the most preferred embodiments of color photographic light-sensitive materials, the dye image forming couplers are incorporated into a silver halide emulsion. These couplers which are incorporated into the emulsion must be rendered nondiffusible (diffusion resistant) in the binder matrix of the emulsion.
The color image forming couplers of the prior art are almost all four-equivalent couplers which require theoretically four moles of silver halide as an oxidizing agent for forming one mole of the dye through the coupling reaction. On the contrary, a two-equivalent coupler having an active methylene group which is substituted with a group capable of being released through the coupling of an oxidized product of an aromatic primary amine developing agent requires only the development of two moles of silver halide for forming one mole of the dye. Since the quantity of silver halide required for forming a dye in the case of a two-equivalent coupler is one half of that required in the case of an ordinary four-equivalent coupler a two-equivalent coupler has many advantages because a thinner light-sensitive layer can be used and the layer can be processed rapidly. In addition, the photographic properties and economy can improved through a reduction in the layer thickness.
Several attempts have been made to convert 5-pyrazolone type couplers which have been conventionally used as a magenta color forming coupler to two-equivalent coupler by substituting a group capable of being split off for one of the hydrogen atoms at the 4-position of the 5-pyrazolone. For example, a pyrazolone in which the 4-position is substituted with a thiocyano group is described in U.S. Pat. Nos. 3,214,437 and 3,253,924, an acyloxy group is described in U.S. Pat. No. 3,311,476, an aryloxy group is described in U.S. Pat. No. 3,419,391, a 2-triazolyl group is described in U.S. Pat. No. 3,617,291, a halogen atom is described in U.S. Pat. No. 3,522,052, and the like.
However, when these 4-position substituted pyrazolone couplers are employed, some disadvantages occur, e.g., a remarkable color fog is produced, coupling reactivity is insufficient, the couplers per se are chemically unstable and change into compounds which can not form the dyes or many difficulties are encountered in the preparation thereof.
5-Pyrazolones in which the 4 -position is substituted with as alkylthio group, an arylthio group or a heterocyclic thio group as described in U.S. Pat. No. 3,227,554 are also known. However, most of these known thio-substituted pyrazolone compounds have the disadvantages that their reactivity with the oxidation products of an aromatic primary amine color developing agent is not suitable, that the mercapto compounds which are formed upon the coupling reaction have such severe photographic effects that they cannot be used in a conventional color photographic light-sensitive material, and that the couplers are chemically unstable.
On the other hand, as couplers which can be used in a color developer solution, four-equivalent couplers have the disadvantage that a larger amount of silver halide is required to obtain a certain color image density, and previously known two-equivalent couplers do not provide preferred results since the couplers are not sufficiently stable in a color developer solution.